1. Field of the Invention
This invention relates to negative impedance bilateral amplifiers and, more particularly, to telephone station repeaters utilizing such amplifiers.
2. Description of the Prior Art
On long telephone loops (exceeding approximately 2800 ohms) it is necessary to provide voice frequency gain at or near the customer end of the telephone loop. Heretofore, this added gain has been supplied by an amplifier contained in the subscriber's telephone handset. One such arrangement is shown in R. E. Holtz et al U.S. Pat. No. 3,963,876, granted June 15, 1976. While this arrangement provides the necessary gain, it requires a relatively expensive amplifier for each extension telephone connected to the same telephone line. Moreover, the amplifier must be sufficiently miniaturized to fit into a telephone handset. Finally, the handset amplifier arrangement introduces a nonstandard telephone set. Such nonstandard items increase the cost of administering the telephone system.
Negative impedance amplifiers are also known which can be inserted in such long telephone loops to provide the needed gain. In order to provide the required impedance-matching to the telephone set, as well as to the long telephone loop, such negative impedance amplifiers have matching circuits utilizing one or more transformers to provide such impedance matching. Moreover, prior designs for such negative impedance amplifiers draw considerable amounts of current from the telephone loop or have to be powered independent of the telephone loop. Due to the small percentage of time during which such amplifier's are actually in use, such power drains place a burden on the operation of the telephone system. Independently powered amplifiers suffer performance loss during power outages.
In addition to the above constraints, it is desirable that a remotely located telephone station repeater require no manual adjustments, be relatively transparent to telephone signaling currents, permit standard line testing, and be susceptible to manufacture at a reasonable cost.